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The presence of quaternary ammonium ions unexpectedly leads to a sizable increase of the kinetic isotope effects in the coupled electron–proton transfer (PCET) reaction of an ascorbate monoanion with the hexacyanoferrate(III) ions in water and this, in “neat” water over-the-barrier coupled electron–proton transfer interaction, entered into tunnelling regime in the presence of the quaternary ions. The kinetic isotope effect between ascorbate monoanion and its 2-OD derivative in the investigated reaction with hexacyanoferrate(III) increased from  k  H / k  D =4.40(0.08) in the reaction in water (in the presence of 8 × 10 3  M NaCl) without the added quaternary ions, to  k  H / k  D =10.08(0.07) in the presence of 1.0 M tetraethylammonium ion, to  k  H / k  D =8.01(0.19) in the presence of 1.0 M of benzyltrimethyl ammonium ion and to  k  H / k  D =7.25(0.02) in the presence of only 0.1 M of tetraethylammonium ion. In contrast,  k  H / k  D =4.06(0.15) has been observed in presence of 0.1 M NaCl. The isotopic ratio of Arrhenius pre-factors  A  H / A  D =0.16(0.01) has been obtained in the presence of only 0.1 M of tetraethylammonium ions and  A  H / A  D =0.10(0.02) in the presence of 0.5 M of the ions. The corresponding observed value is  A  H / A  D =0.23(0.02) in the presence of 0.5 M of benzyltrimethylammonium ions and  A  H / A  D =0.35(0.06) in the presence of 0.5 M tetramethylammonium ions. The differences in the enthalpies of activation Δ Δ H ‡  between D 2 O and H 2 O all are well beyond the semiclassical value of 5  .1 kJ/mol for the difference between zero-point energies  E  o  D – E  o  H  for dissociation of an O–H bond. The observed tunnelling phenomena point to a role of dynamics of the transition configuration of the PCET process, coupled with dynamics of hydrogen-bonded structures related to the solvent shell of the reactive configuration and its environment including the nearby quaternary ammonium ions.

zeitschrift für physikalische chemie

Sizeable Increase of Kinetic Isotope Effects and Tunnelling in Coupled Electron–Proton Transfers in Presence of the Quaternary Ions. PCET Processes and Hydrogen Tunnelling as a “Probe” for Structuring and Dynamical Phenomena in Water Solution